The generator stator core is the largest single component in the train of a turbine generator set. The stator cores are generally manufactured from thousands of laminations of relatively thin steel plates which are stacked, pressed and clamped together into the large cylindrical form of the stator core. The clamping is necessary to accommodate variations in thickness of the stock steel plate laminations, commonly referred to as crowns. Improperly clamped laminations can result in plate vibration during generator operation, which results from magnetic impulses or core elliptical dilation.
Typically, the stator core is assembled from the steel plates directly at the final installation site. However, the large size of the stator core and the need for proper clamping results in stator core manufacturing difficulties, including generous floor space and high crane requirements. The manufacture of stator cores via the traditional methods result in manufacturing lead time and other associated manufacturing difficulties. For example, if the core is stacked directly in the stator frame, the frame must be delivered to the site before any manufacturing steps can occur. Additionally, intermediate core pressing equipment is needed to press and clamp the steel plates together at incremental lengths. If, on the other hand, the stator core is manufactured in an external fixture, the external fixture itself adds to the manufacturing costs and requires additional floor space on site and still requires the use of heavy cranes.
U.S. Pat. No. 5,875,540 by Sargeant, which is incorporated herein by reference, overcame some of the problems with the prior art by first assembling a number of laminations into a distinct set, referred to as a donut, and then stacking these donuts to form a stator core. This saved great amounts of time over assembling the laminations individually, and produced a stator core with less flaws.
When the individual laminations, or the set of laminations in a donut, are installed into a core, they engage what are referred to as keybars. Keybars are essentially rods that run the internal length of the stator core and provide a hook-in spot for the laminations. FIG. 1 illustrates a stator frame that is empty of any laminations. The keybars 6 run the internal length of the stator frame 2, and are generally attached to the frame through stator support rings 4.
The size of laminations and resulting stator core can vary, but a typical stator core lamination has a weight of 3.6 lbs (1.6 kg). The results in a weight per inch of core of 1530 lbs/in (1740 kg/cm). For a core length of 225 inches (563 cm), the total weight will be approximately 344,250 lbs (156,477 kg).
In the prior art, when generators need their cores replaced, the only way to accomplish this has been to strip out the old core and then re-stack the new core lamination by lamination. Since the generators are large, immobile objects, each lamination needs to be horizontally inserted into position by hand. This is a very time consuming effort, which requires that the generator be off-line for long periods of time. What is needed is a better way of adding a new core to an existing generator frame.